Updated January 26, 2018 at 11:36 AM;Posted January 26, 2018 at 9:00 AM

David Robertson #30 of the New York Yankees delivers a pitch during the eighth inning against the Cleveland Indians in game three of the American League Division Series at Yankee Stadium on October 8, 2017 in New York City. (Mike Stobe/Getty Images)

Baseball's offseason is a puzzle that's worked on simultaneously by 30 general managers and a slew of agents.

While it's impossible to predict how every roster will look when opening day arrives in late March, themes become clear as any winter moves along. For the Yankees in 2018, one thing is almost certain: Blowing past the luxury tax for, say, Yu Darvish or Mike Moustakas isn't going to happen.

So while the Yankees have around $35M in payroll space to work with, factors (insurance, saving money for an in-season trade) make signing a star to a contract worth $20 million (or more) a difficult task.

Instead, sources say the Yankees' tentative plan allows for another $15 million in spending this offseason -- unless Cashman frees up additional payroll by trading reliever David Robertson or outfielder Jacoby Ellsbury.

To do something extensive -- like signing Moustakas -- would involve sending away a large contract, and Ellsbury's is a popular choice among Yankees supporters. But it's been difficult for the Yanks to move Ellsbury, given his disappointing offensive numbers and large salary.

Robertson, 32, pitched to a 1.84 ERA in 2017. That work included an incredible 1.03 ERA across 30 appearances with the Yankees after a midseason trade sent the veteran back to the Bronx. Robertson is due $13M in 2018--a bargain for one of baseball's best relief arms.